portable tv signal reception

hi ppl!

I am in the process of building a mobile computer in my Skoda 120L. It will use one of those small portable TVs with AV input as a screen. Right now, only the TV with its power supply is installed in the car. My problem is: I would like to use it as a TV right now, but there is very little reception inside the car, presumably because the body 'shields' the signal. Is there any way I can cheaply increase the reception? Is it a good idea to connect the rod antenna fixed to the tv with the car body or the radio antenna?